Stable Planar Six-π-Electron Six-Membered N-Heterocyclic Carbenes

Published on Web 06/30/2005

Stable Planar Six-π-Electron Six-Membered N-Heterocyclic Carbenes with Tunable Electronic Properties Carsten Pra¨sang, Bruno Donnadieu, and Guy Bertrand* UCR-CNRS Joint Research Chemistry Laboratory (UMR 2282), Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403 Received May 6, 2005; E-mail: [email protected].

The last 15 years have seen tremendous advances in carbene chemistry that transformed these electron-deficient species1 into the realm of isolable compounds2 and powerful tools for synthetic chemists. Despite the existence of several families of stable carbenes, it has to be recognized that, so far, only the cyclic diamino carbenes (NHCs) have found numerous applications. NHCs are excellent catalysts on their own,3 and there is increasing experimental evidence that NHC-metal catalysts surpass their phosphinebased counterparts in both activity and scope of application.4 Having said that, it is clear that the success of phosphorus- and nitrogenbased ligands was, and is still, due to their enormous structural diversity. So far, the structures of stable NHCs available have been largely restricted to saturated and unsaturated five-membered heterocycles A5 and B,6 the only exceptions being carbenes C-F7 (Scheme 1). Moreover, it has been shown that free acyclic carbenes, including diamino carbenes,8 are far more fragile than NHCs and are poor ligands for transition metal complexes.9 Therefore, it is of interest to design new stable cyclic diamino carbenes possessing different scaffolds and electronic structures. Borazines G, discovered by Stock10 in 1926, and currently used for the fabrication of advanced material based on boron nitride,11 constitute a family of extremely stable heterocycles. Because they are isoelectronic to benzene, they are often regarded as the archetypical example of an inorganic aromatic compound, although the extent of aromaticity of G in relation to that of benzene is still debated.12 Since boranes are isoelectronic with both carbocations and carbenes, it was reasonable to believe that the mixed organic/ inorganic heterocycles H and I would be quite stable. Importantly, the lone pairs of the nitrogen atoms, adjacent to the carbene center of I, can interact both with the carbene and with the boron vacant orbitals. Varying the nature of the substituents at the boron centers, and thus modifying their Lewis acidity, should allow for the preparation of carbenes with quasi-identical steric demands, but different electronic properties; this would permit a better understanding of the factors governing the catalytic activity of the ensuing complexes. Derivative 4a13 was obtained in one step (83% yield) by treatment of N-silylated formamidine 1 with compound 2. To replace the amino group at the boron centers by a weaker π-donor substituent, a different synthetic approach has been designed; indeed, 1,3dibromo-1,3-diboranes of type 2 are only readily available with amino substituents. Derivative 3 was prepared according to a known procedure.14 Then, deprotonation with n-butyllithium and subsequent treatment with the desired dibromoborane afforded derivatives 4b,c13 in 74 and 71% yield, respectively. The influence of the nature of the boron substituents is clearly apparent in the 1H NMR chemical shift of the iminium proton: 8.8 (4a), 9.4 (4b), 9.7 (4c) ppm. Addition at -78 °C of lithium tetramethyl piperidine to a THF solution of derivatives 4a-c cleanly afforded carbenes 5a10182

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Scheme 1

Scheme 2

c, which were isolated after recrystallization from hexanes as lightyellow crystals13 (Scheme 2). Both types of heterocycles 4 and 5 are perfectly stable at room temperature, in solution, and in the solid state. The single-crystal X-ray diffraction study15 of 4c and 5c (Figure 1) reveals, in both cases, the expected planar six-membered ring skeleton with a propeller-like arrangement of substituents. The cyclohexyl rings feature the most protecting conformation for the electron-deficient carbon center. As always observed, when comparing the geometric parameters of NHCs and those of their NHC(H+) precursors, the N2-C1-N3 angle for carbene 5c (114.5°) is more acute than that for cation 4c (122.8°). For the cation 4c, the rather short endocyclic N-C distances (1.33 and 1.32 Å) and the somewhat longer N2B1 and N3-B2 bond distances (1.46 and 1.47 Å) compared to those of B1-N1 and B2-N1 (1.42 and 1.43 Å) suggest that the nitrogen lone pairs interact primarily with the carbon center. Indeed, in borazines, B-N distances are in the range of 1.42-1.44 Å.16 In other words, the six π-electrons are divided into two separated allyl moieties: a cationic four π-electron NCN and a neutral four π-electron BNB system. In contrast, carbene 5c exhibits five B-N 10.1021/ja052987g CCC: $30.25 © 2005 American Chemical Society

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and 6c (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. References

Figure 1. Thermal ellipsoid diagram (50% probability) of 4c (left) and 5c (right) (H atoms are omitted). Selected bond distances (Å) and angles (°): 4c C1-N2 1.323(4), C1-N3 1.328(4), N2-B1 1.467(4), N3-B2 1.460(4), B1-N1 1.428(5), B2-N1 1.422(5), N2-C1-N3 122.8(3); 5c C1N2 1.3664(18), C1-N3 1.3634(18), N2-B1 1.4442(19), N3-B2 1.4449(19), B1-N1 1.4278(19), B2-N1 1.4356(19), N2-C1-N3 114.45(12).

bond lengths, which are about equal (1.43-1.44 Å), and relatively long CN bond distances (1.36-1.37 Å), indicating a more delocalized π-system. To test the coordination ability of derivatives 5, carbene 5c was treated at -78 °C with half an equivalent of bis(µ-chlorocyclooctadienerhodium). After workup, complex 6c was isolated as highly thermally stable single crystals. Not surprisingly, the value of the Ccarbene-Rh bond length for 6c (2.05 Å) is within the typical range for analogous complexes bearing classical NHC ligands A-D (2.00-2.06 Å).17 To compare the electron-donating ability of carbenes 5a-c, the corresponding cis-[RhCl(CO)2(5)] complexes 7a-c were prepared by treatment of 6c with CO at room temperature, or alternatively by addition of carbenes 5a-c to half an equivalent of bis(µ-chlorodicarbonylrhodium). The average value of the carbonyl stretching frequencies [νav(CO): 2029 (7a), 2033 (7b), 2038 (7c), cm-1] clearly indicate that the donor power of carbenes 7 decreases in the order 7a > 7b > 7c, following the donor ability of the boron substituents. It is important to note that (i) a difference of only 3 cm-1 was observed between the νav(CO) values of saturated A (2038) and unsaturated B (2041) NHCs; (ii) the donor power of 7a and 7b seems to be higher than those of A and B, which is in agreement with the higher electropositivity of boron compared to that of carbon. In conclusion, carbene analogues of borazines are readily available as highly thermally stable species [mp 176 °C without decomposition (7c)]. They give rise to stable transition metal complexes. Importantly, their electronic properties can readily be tuned by varying the nature of the boron substituents. Their efficiency as organic catalysts and as ligands for transition metal catalysts is currently under investigation. Acknowledgment. We are grateful to RHODIA and the NIH (R01 GM 68825) for financial support of this work, and to the Alexander von Humboldt Foundation (C.P.). Acknowledgment. Full experimental and spectroscopic data for all new compounds, and X-ray crystallographic data for 4c, 5c,

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